We run a large computer hardware department, the largest in our market space, so I can tell you why so many Point of Sales systems ours and others fail.
By far, the biggest problem is all this software that people keep putting on software some of it is quite good, but it's just too big for their computers. Then some people have gone down the path of tweaking their system to try to get what is a best marginal performance boost, but often they have done harm to their system. Then add some slow antivirus software, which is designed for a modern computer which there is not. Now add the spyware and viruses which all of us pick up and which no antivirus software can stop all of them. Then over time their files become fragmented. Soon their system is running real slow, and sometimes they have to call us.
The fact is that most current software is designed for the latest computers, as most of the buyers use these computers. If your computer is not reasonably modern and fast, what you are doing is putting on a problem. Removing it often does not work 100% as they repeatedly leave a residue behind. Most good software like ours is designed to juggle threads and optimisation and you are not going to pick up much speed there.
Finally few decent point of sale systems are off-the-shelf packages; they have been carefully crafted to run as you require so playing around with them is often dangerous too.
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